Nanostructured sensor for high temperature applications

ABSTRACT

A gas sensor utilizes nano-sized CeO 2  and doped CeO 2  particles for detecting NO, NO2 and also for studying the cross sensitivity of oxygen, un-burnt hydrocarbons, CO and CO 2 . Nano-crystalline powders of CeO 2  and doped CeO 2  are employed to configure thin films on Platinum comb type electrodes preformed on alumina substrates. Various catalytic oxides are employed to convert the NO to NO 2  to get equal response to NOx gas. Gas sensing properties are measured using a dynamic chamber with a constant flow of air and NOX gas in required percentage in nitrogen gas.

TECHNICAL FIELD

Embodiments are generally related to gas sensors. Embodiments are alsorelated to the field of NO_(x) sensors using nano-crystalline CeO₂.Embodiments are additionally related to CeO₂ (MOS) NO_(x) sensors forhigh temperature applications.

BACKGROUND OF THE INVENTION

The role of gases and the measurement of their concentration have alwaysreceived wide spread applications in many fields of science andtechnology. This has resulted in an increasing demand for small scalesolid-state sensors. NO_(x) sensors for automotive exhaust gasenvironments are of great interest because of high expectations ofnanostructured materials and ever increasing demands on emission controllegislations. The sensitivity of a gas sensor is defined as the ratio ofthe resistance of the sensor element in air to the resistance of thesensor element in the test gas atmosphere (S=R_(air)/R_(gas)).

Many processes and devices have been used for sensing exhaust gases fromautomobile engines. NO_(x) is one of the unwanted exhaust gases whichpollute the environment. NO_(x) is a term used to describe the totaloxides of nitrogen, which are commonly estimated from the measured NO,based on the assumption that the total NO_(x) is (combination of NO andNO2 with varying concentrations depend upon the engine conditionsranging from 40% to 5% for NO2 and 60% to 95% of NO). This assumption isgenerally acceptable when combustion exhaust gases are measured at theoutlet of a combustion system and the oxygen concentration is low. Ifthe measurement is made at the exhaust outlet or in the atmosphere, theNO₂ is likely much higher than total 5% of the total NO_(x).

Measurement of NO and NO₂ is recommended for accurate total NO_(x)formation. NO_(x) is important to measure because of reactions involvingvolatile organic compounds (VOCs) with nitrogen oxides (NO_(x)) in thepresence of sunlight form ozone in the atmosphere. Ground-level ozoneand NOx for example, causes throat irritation, congestion, chest pains,nausea and labored breathing. Ozone can also aggravate respiratoryconditions, such as chronic lung and heart diseases, allergies andasthma. Additionally, Ozone ages the lungs and may contribute to varioustypes of lung diseases.

NO_(x) is found in emissions from aircraft, automobiles and industrialfactories and contributes to the production of acid rain, smog, and thedepletion of the ozone layer. With the increase in the number ofvehicles traveling the earth, the amount of NO_(x) produced is alsoincreasing, thereby causing a dangerous situation for the environment.Therefore, a reliable NO_(x) sensor to monitor and control emissionswhile exposed to the harsh conditions is needed.

The development of gas sensor devices with optimized selectivity andsensitivity has been gaining prominence in recent years. The use ofsemiconductor fabrication line is the preferred manufacturing processbecause of the potential to reduce cost. However fundamental materialsand processing issues which are critical for high performance gassensors need to be addressed. Among the new technologies anano-crystalline material offers immense promise for improvedsensitivity.

Nano-crystalline materials are currently receiving a great deal ofattention due to their unique physical properties, which derive fromtheir nanometer scaled sizes. In nano-sized materials, for example, thesurface to bulk ratio is much greater than coarse materials, so that thesurface properties become paramount, which makes them particularlyappealing in applications, such as gas sensors, where nano-sizedproperties can be exploited. Grain size reduction, for example, is oneof the main factors for enhancing the gas sensing properties ofsemiconducting oxides. It is believed that improved sensing technologiescan therefore be configured and developed by taking advantage of recentadvances in nano-sized materials.

BRIEF SUMMARY

The following summary is provided to facilitate an understanding of someof the innovative features unique to the embodiments disclosed and isnot intended to be a full description. A full appreciation of thevarious aspects of the embodiments can be gained by taking the entirespecification, claims, drawings, and abstract as a whole.

It is, therefore, one aspect of the present invention to provide for animproved NO_(x) sensor to monitor and control emissions when exposed toharsh conditions.

It is another aspect of the present invention to provide for a gassensor that utilizes nano-sized CeO₂ particles to detect NO_(x) andstudy the cross sensitivity for oxygen, unburnt hydrocarbons, CO andCO₂.

The aforementioned aspects and other objectives and advantages can nowbe achieved as described herein. CeO₂ nano-crystalline powders aresynthesized by employing sol-gel, co-precipitation as well as chemicalvapor synthesis (CVS). Such powders are used for configuring thin filmsof CeO₂ on platinum inter digital comb type electrodes performed onalumina substrates to form a sensor thereof. On the other side of thesensor, a platinum heater is provided to maintain the sensor at hightemperatures. The nano powders obtained by the above said methods aredispersed on these substrates by dip coating, or screen printing byadding the appropriate binders for making thin and thick films.Dispersing the nano-crystalline powders in organic solvents and byemploying electrophoresis techniques, thin films also can be fabricated.

Sintering is carried out to enhance the adherence of these films to thesubstrate. Thick films are also prepared by using screen printingtechniques of CeO₂ in association with an appropriate binder andsintered at higher temperatures. Such films can be impregnated with 2%platinum particles. The gas sensing properties of NOx can be carried outusing a test apparatus, which can indicated that the sensitivity for2500 pm of NO and NO₂ is approximately 250% and the response andrecovery time are less than four seconds for the same concentration. Theuniqueness of the disclosed technique and device stems from the controlof the particle size and shape. Especially with chemical vaporsynthesis, the particle size can be controlled up to 8 to 10 nm. Higherparticle sizes are also easily achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally-similar elements throughout the separate viewsand which are incorporated in and form a part of the specification,further illustrate the embodiments and, together with the detaileddescription, serve to explain the embodiments disclosed herein.

FIG. 1 illustrates a gas sensor testing apparatus, which can beimplemented in accordance with a preferred embodiment;

FIG. 2 illustrates side view of the CeO₂ NOx gas sensor which can beimplemented in accordance with an alternative embodiment; and

FIG. 3 illustrates a perspective view of the back side of the sensor onwhich a platinum heater is provided on the substrate in accordance withthe present embodiment.

FIG. 4 illustrates a flowchart of operations depicting logicaloperational steps for the preparation of nano-crystalline CeO₂ and dopedCeO2 coating, in accordance with a preferred embodiment;

FIG. 5 illustrates a flowchart of operations depicting logicaloperational steps for the detection of NO_(x) gases using CeO₂NO_(x) gassensor, in accordance with a preferred embodiment; and

FIG. 6 illustrates a side view of a sensor, which can be implemented inaccordance with an alternative embodiment.

DETAILED DESCRIPTION

The particular values and configurations discussed in these non-limitingexamples can be varied and are cited merely to illustrate at least oneembodiment and are not intended to limit the scope thereof.

Referring to FIG. 1, a gas sensor testing apparatus 100 is illustrated,which can be implemented in accordance with a preferred embodiment. Thegas sensor testing apparatus 100 generally includes two gas cylindertanks 110 and 120. The gases NO_(x) filled in cylinder 120 and dry airfilled in 110 respectively flow from gas cylinder tanks 120 and 110, andare allowed to pass through a mass flow controller 130 to adjust theflow rate. Apparatus 100 further includes a two way gas valve 140. Byadjusting the two way gas valve 140, NOx and dry air can be selectivelypassed on to a sensor 160 that detects the gas content.

Current voltage properties can be measured using a high voltage source170 (e.g., a power supply). A stand 150 can also be provided upon whichthe two-way gas valve 140 and gas sensor 160 are connected. Theconductance of the sensor 160 can be measured with a digital multimeter180 that is connected electrically to the high voltage source 170 andalso to a computer 190. The change in resistance can be simultaneouslymonitored by the digital multimeter 180. The apparatus 100 also includesthe control computer 190, which is generally operable to control andmanage the overall operation of the testing apparatus 100.

Referring to FIG. 2 a side view of a CeO₂ NOx gas sensor element 200 isillustrated, which can be implemented in accordance with a preferredembodiment. In FIGS. 2 and 3( a)-3(b), identical or similar parts orelements are generally indicated by identical reference numerals. Notethat the CeO₂NOx gas sensor element 200 depicted in FIG. 2 can beadapted for use with the sensor 160 depicted in FIG. 1. The gas sensor160 functions based on the fact that the changes of the oxide filmresistance result from the reactions of the gases on the surface of thefilm 220 The gas sensor 160 includes the CeO₂NOx gas sensor element 200,which is composed of a platinum heater 240 formed in association with analumina ceramic substrate 230. An interdigital comb of platinumelectrodes 210 can be formed on one side of the alumina ceramicsubstrate 230. One or more thin films 220 of CeO₂ can be fabricated onthe platinum electrodes 210 by electrophoresis. On the other side of thesensor element 200, the platinum heater 240 can be provided to maintainthe sensor element 200 at high temperatures.

Referring to FIG. 3A, a front view of the CeO₂ NOx gas sensor element200 is depicted, including a CeO₂ coating, in accordance with apreferred embodiment. The sensor platinum electrode 210 is generallyprovided in the context of an inter-digital comb structure, whichmaintains the resistance in an easily measurable range. The sensingmechanism of sensor element 200 is based on the electrofilic adsorptionof NO_(x) gas on the semi conducting oxide material (i.e., CeO₂) of thefilms 220. The change in conductivity of the sensor element 200 can bemeasured and calibrated with known concentrations.

Referring to FIG. 3B a back view of CeO₂ NOx gas sensor element 200including one or more platinum heaters is illustrated in accordance witha preferred embodiment. On the back side of the substrate 230, theplatinum heater 240 can be mounted in order to maintain the sensorelement 200 at an appropriate operating temperature. A chemical reactionoccurs when combustible gas reaches the sensing element 200. This actionincreases the temperature of the element 200, such that the heat istransmitted to the platinum heater 240.

A heating element is used to regulate the sensor temperature, since thefinished sensors exhibit different gas response characteristics atdifferent temperature ranges. This heating element can be a platinum orplatinum alloy wire, a resistive metal oxide, or a thin layer ofdeposited platinum. The sensor element 200 can then be processed at aspecific high temperature, which determines the specific characteristicsof the finished sensor. In the presence of gas, the metal oxide causesthe gas to dissociate into charged ions or complexes, which results inthe transfer of electrons. The built-in platinum heater 240 thus heatsthe metal oxide material to an operational temperature range that isoptimal for gas to be detected, and can optionally be regulated andcontrolled by a specific circuit. This specific circuit can be a chip(Application-Specific Integrated Circuit, ASIC) which can control sensortemperature through an independent measurement and heating mechanism ofthe micro heater present inside the chip.

Referring to FIG. 4 a flowchart of operations is illustrated depictinglogical operational steps for the preparation of a nano-crystalline CeO₂coating, in accordance with a preferred embodiment. As indicated atblock 310, CeO₂ nano crystalline powders can be synthesized by employingsol-gel, precipitation as well as chemical vapor synthesis. Interdigital comb type of Platinum electrodes are generally formed on oneside of an alumina ceramic substrate, as indicated at block 320, byusing a screen printing technique. Thereafter, as indicated at block330, on the other side of the sensor, a Platinum heater can be providedto maintain the sensor at high temperatures. Nano-crystalline powdersare generally dispersed in organic solvents and by employingelectrophoresis or a dip coating technique as illustrated at block 340,the thin films can be fabricated.

Next, as indicated at block 350, a sintering operation can be carriedout to enhance the adherence of these films to the substrate. Thedifficulty of sintering of CeO₂ (as the sintering temperature of CeO₂ isbeyond 1600 C) is solved by adding inorganic binders mixing (5%) withCeO₂. Thick films can also be prepared using a screen printing techniqueof CeO₂ with an appropriate binder and sintered at high temperatures.The cross sensitivity of other gases (e.g., hydrocarbons, CO, CO₂ etc)can be checked thoroughly by adding a catalytic metal such as platinum360 as depicted block 360. The cross sensitivity can thus be reduced tospecified limits.

Referring to FIG. 5 a flowchart 400 of operations depicting logicaloperational steps for the detection of NO_(x) gases using a CeO₂ NO_(x)gas sensor is illustrated, in accordance with a preferred embodiment. Asdepicted at block 410, the exhaust gas can be absorbed on semiconducting oxide material. Thereafter, as indicated at block 420,catalytic metals can be applied to avoid cross-sensitivity andinterference from other gases. Next, NO_(x) gas can be sensed on asemi-conducting oxide material (CeO₂) based on electrophilic adsorption,as depicted at block 430. Thereafter, as depicted at block 440, changesin the conductivity of the semi-conducting oxide material can bemeasured. A Cerium Oxide (CeO₂) NOx sensor can then be calibrated withknown concentration, as depicted at block 450.

In such an application, the sensor may produce a sensitivity of 200%with respect to a change of resistance for 2500 ppm of NO and NO₂. Theparticle size effect begins to occur below 50 nm with an order ofmagnitude increase in sensitivity for particles in 20 to 30 nm range.This particle size effect is due, in part, to an increase in the surfacearea since. In this range, a large fraction of the atoms (e.g., up to50) are generally present at the surface or the interface region so thatthe structure and properties are different from that of the bulkmaterial. However, the main effect is associated with the depth of thesurface space charge region affected by gas adsorption in relation tothe particle size. By employing a sol-gel process, for example, a 20 nmsize of CeO₂ can be obtained. Such nano-powders are preferably mixedwith adequate (5 to 10 wt %) amounts of ethylene glycol and the paste isthen applied on to the platinum electrodes on an alumina substrate. Theother method employed is by adding an appropriate amount (5 to 20% bywt) of binder ink making the printable ink for screen printing to beused for thick film sensor production.

The sensor described herein is very simple to fabricate and possesses afast response and recovery for the NO_(x) gas because of the presence ofthe nano-sized particles. Such benefits can be achieved due to the largesurface area and reactive nature of the nano-crystalline powders. Thecost of the sensor is relatively inexpensive, because large scalemanufacturing processes such as screen printing can be employed. Theelectronics used to measure conductivity change are also much lesscomplex and generally inexpensive. Cerium oxide in a thick film form,for example, can also be prepared using nanopowders and tested forNO_(x) sensing. The methodology and device disclosed herein thereforeuses nano-sized CeO₂ particles to detect NO and NO₂ and employsnano-crystalline powders of CeO₂ to configure thin films on Platinumcomb type electrodes preformed on alumina substrates.

FIG. 6 illustrates a side view of a sensor 500, which can be implementedin accordance with an alternative embodiment. Sensor 500 generallyincludes a thick platinum film heater 550 formed in association with asubstrate 540, which can be configured from alumina or ceramic. Aninter-digital comb of electrodes 510 can be formed on one side of thealumina or ceramic substrate 540. Electrodes 510 can be formed fromplatinum. A thick film 530 of sensing element Ce_((1-x)) T_(x) O_((2-y))can be fabricated on the electrodes 510 by electrophoresis or screenprinting, depending upon design considerations. A thick film 520 ofcatalyst material can be fabricated on the sensing element 530 (i.e.,Ce_((1-x)) T_(x) O_((2-y))). On the other side of the sensor element500, the platinum film heater 550 can be provided to maintain the sensorelement 500 at high temperatures. The configuration of sensor 500generally permits a catalyst material 520 or a combination of catalysts(e.g., WO₃, MoO₃, XWO₄, X₃WO₅, X₃W₂O₉ (x=Ca, Ba, Sr), YMoO₄, Y₂MoO₅,Y₃Mo₃O₉ (Y=Ca, Ba, Sr), to be used to convert the NO to NO₂ and sensethe NOx gas of any combination of NO and NO₂ and to provide the sameoutput. Sensor 500 thus constitutes an alternative version of a CeO₂NO_(x) sensor.

It will be appreciated that variations of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A method of providing a CeO₂ NO_(x) gas sensor, comprising: providinga substrate; forming a pair of interdigital electrodes on a side of saidsubstrate and a heater another side of said substrate; and applying acoating of nano-crystalline powders of doped CeO₂ on said pair ofinterdigital electrodes formed on said substrate in order to form a CeO₂NO_(x) gas sensor.
 2. The method of claim 1 further comprisingconfiguring said pair of interdigital electrodes in a comb-typeconfiguration upon said side of said substrate and said another side ofsaid substrate.
 3. The method of claim 1 wherein said substratecomprises alumina or such similar substrates of low thermal expansioncoefficient.
 4. The method of claim 1 wherein said substrate comprises aceramic material or a semi-conducting material
 5. The method of claim 1further comprising configuring a heater upon said substrate wherein saidheater and said pair of interdigital electrodes comprise platinum. 6.The method of claim 1 further comprising providing a catalyst materialin order to convert NO to NO₂ and thereby detect NOx gas for anycombination of NO and NO₂ by said CeO₂ NOx gas sensor and provide a sameoutput thereof.
 7. The method of claim 5 further comprising forming saidheater utilizing a screen printing on said substrates following asintering at a temperature of 1000° C.
 8. The method of claim 1 whereinapplying a coating of nano-crystalline powders of CeO₂ on said pair ofinterdigital electrodes formed on said substrate, further comprises:synthesizing a nano-crystalline powder by employing a sol-gel and achemical vapor synthesis; dispersing said nano-crystalline powder in anorganic solvent; employing a dip coating or an electrophoresis operationto fabricate at least one thin film for deposition upon said substrate;carrying out sintering operation to enhance an adherence of said atleast one thin film to said substrate; and adding a catalytic mesh ofnoble metal.
 9. The method of claim 7 wherein said sintering operationis carried out by adding an inorganic binding mixture.
 10. The method ofclaim 7 wherein said nano-crystalline powder is mixed with an equalamount of ethylene glycol and a paste applied thereafter to say on tosaid pair of interdigital electrodes.
 11. A method of providing a CeO₂NO_(x) gas sensor, comprising: providing a substrate; forming a pair ofinterdigital electrodes on a side of said substrate and a heater anotherside of said substrate; configuring said pair of interdigital electrodesin a comb-type configuration upon said side of said substrate and saidanother side of said substrate; and applying a coating ofnano-crystalline powders of doped CeO₂ on said pair of interdigitalelectrodes formed on said substrate in order to form a CeO₂ NO_(x) gassensor.
 12. The method of claim 11 wherein said substrate comprisesalumina or such similar substrates of low thermal expansion coefficient.13. The method of claim 11 wherein said substrate comprises a ceramicmaterial or a semi-conducting material
 14. The method of claim 11further comprising: providing a catalyst material in order to convert NOto NO₂ and thereby detect NOx gas for any combination of NO and NO₂ bysaid CeO₂ NO_(x) gas sensor and provide a same output thereof; andforming said heater utilizing a screen printing on said substratesfollowing a sintering at a temperature of 1000° C.
 15. The method ofclaim 11 wherein applying a coating of nano-crystalline powders of CeO₂on said pair of interdigital electrodes formed on said substrate,further comprises: synthesizing a nano-crystalline powder by employing asol-gel and a chemical vapor synthesis; dispersing said nano-crystallinepowder in an organic solvent; employing a dip coating or anelectrophoresis operation to fabricate at least one thin film fordeposition upon said substrate; carrying out sintering operation toenhance an adherence of said at least one thin film to said substrate;and adding a catalytic mesh of noble metal.
 16. A CeO₂ NO_(x) gas sensorapparatus, comprising: a substrate; a pair of interdigital electrodesconfigured on a side of said substrate and a heater another side of saidsubstrate; and a coating of nano-crystalline powders of doped CeO₂applied on said pair of interdigital electrodes formed on said substratein order to form a CeO₂ NO_(x) gas sensor.
 17. The apparatus of claim 16wherein said pair of interdigital electrodes are arranged in a comb-typeconfiguration upon said side of said substrate and said another side ofsaid substrate.
 18. The apparatus of claim 16 wherein said substratecomprises alumina or such similar substrates of low thermal expansioncoefficient.
 19. The apparatus of claim 16 wherein said substratecomprises a ceramic material or a semi-conducting material
 20. Theapparatus of claim 16 further comprising a heater configured upon saidsubstrate wherein said heater and said pair of interdigital electrodescomprise platinum.